The mystery of the “slow” solar wind revealed by the Solar Orbiter mission

Scientists have taken a step closer to identifying the mysterious origins of the “slow” solar wind, using data collected during the Solar Orbiter spacecraft’s first close trip to the sun.

The solar wind, which can travel at hundreds of kilometers per second, has fascinated scientists for years and new research published in Natural astronomyfinally highlights his training.

Solar wind describes the continuous flow of charged plasma particles from the sun into space, with wind traveling at more than 500 km per second called “fast” and less than 500 km per second described as “slow”.

When this wind hits Earth’s atmosphere, it can cause stunning auroras that we know as the Northern Lights. But when larger amounts of plasma are released, in the form of a coronal mass ejection, it can also be dangerous, causing significant damage to satellites and communications systems.

Despite decades of observations, the sources and mechanisms that release, accelerate, and transport solar wind plasma away from the sun and into our solar system are not well understood, particularly the slow solar wind.

In 2020, the European Space Agency (ESA), with support from NASA, launched the Solar Orbiter mission. In addition to capturing the closest and most detailed images of the sun ever taken, one of the mission’s main goals is to measure and relate the solar wind to its area of ​​origin on the sun’s surface.

Described as “the most complex scientific laboratory ever sent to the sun”, there are ten different scientific instruments aboard Solar Orbiter, some in situ to collect and analyze samples of the solar wind as it passes the spacecraft, and other remote sensing instruments. instruments designed to capture high-quality images of activity on the sun’s surface.

By combining photographic and instrumental data, scientists were able, for the first time, to more clearly identify the origin of the slow solar wind. This helped them understand how it is able to leave the sun and begin its journey toward the heliosphere, the giant bubble around the sun and its planets that protects our solar system from interstellar radiation.

Dr Steph Yardley from the University of Northumbria in Newcastle upon Tyne led the research and explains: “The variability of solar wind fluxes measured in situ on a spacecraft close to the sun provides us with a lot of information about their sources, and well than previous studies If we traced the origins of the solar wind, it was done much closer to Earth, at which point this variability disappeared.

“As Solar Orbiter travels so close to the sun, we can capture the complex nature of the solar wind to get a much clearer picture of its origins and how this complexity is driven by changes in different source regions.”

The difference between fast and slow solar wind speeds is thought to be due to the different areas of the solar corona, the outermost layer of its atmosphere, from which they originate.

The open corona refers to regions where magnetic field lines anchor to the Sun at one end and extend into space at the other, creating a highway for solar matter to escape into the Sun. ‘space. These areas are cooler and would be the source of the fast solar wind.

Meanwhile, the closed corona refers to regions of the sun where its magnetic field lines are closed, meaning they are connected to the solar surface at both ends. These can be seen as large loops of light that form over magnetically active regions.

Sometimes these closed magnetic loops break, providing a brief opportunity for solar matter to escape, in the same way it does through open magnetic field lines, before reconnecting and forming a closed loop. This usually occurs in areas where the open and closed crown meet.

One of Solar Orbiter’s goals is to test a theory that the slow solar wind originates from the closed corona and is able to escape into space through this process of breaking and reconnecting magnetic field lines.

The scientific team was able to test this theory by measuring the “composition” of solar wind flows.

The combination of heavy ions contained in solar matter differs depending on its origin; the warmer, closed corona compared to the open, cooler corona.

Using the instruments on board Solar Orbiter, the team was able to analyze the activity taking place on the surface of the sun, then compare it to the solar wind flows collected by the spacecraft.

Using images of the sun’s surface captured by Solar Orbiter, they were able to determine that the slow winds were coming from an area where the open and closed coronas met, proving the theory that the slow wind is capable of escaping closed magnetic field lines. through the process of breaking and reconnecting.

As Dr Yardley, from the Solar and Space Physics Research Group at Northumbria University, explains: “The varying composition of the solar wind measured by Solar Orbiter was consistent with the change in composition between corona sources.

“Changes in the composition of heavy ions as well as electrons provide strong evidence that not only is the variability due to different source regions, but it is also due to reconnection processes occurring between the closed and open loops of the crown.”

ESA’s Solar Orbiter mission is an international collaboration, with scientists and institutions from around the world working together, bringing specialist skills and equipment.

Daniel Müller, ESA Solar Orbiter project scientist, said: “From the beginning, one of the central goals of the Solar Orbiter mission has been to link dynamic events on the sun to their impact on the plasma bubble. surrounding the heliosphere.

“To achieve this, we need to combine remote observations of the sun with in situ measurements of the solar wind as it passes in front of the spacecraft. I am extremely proud of the entire team who managed to achieve these complex measurements.

“This result confirms that Solar Orbiter is capable of establishing strong connections between the solar wind and its source regions on the solar surface. This was a key objective of the mission and paves the way for us to study the “origin of the solar wind in unprecedented detail.”

Among the instruments aboard Solar Orbiter is the Heavy Ion Sensor (HIS), developed in part by researchers and engineers at the University of Michigan Space Physics Research Laboratory in the Department of Science and Technology. of climate and space engineering. The sensor was designed to measure heavy ions present in the solar wind, which can be used to determine where the solar wind is coming from.

“Each region of the Sun may have a unique combination of heavy ions, which determines the chemical composition of a solar wind stream.

“Because the chemical composition of solar wind remains constant as it propagates through the solar system, we can use these ions as a fingerprint to determine the origin of a specific flow of solar wind in the lower part of the solar atmosphere .” said Susan Lepri, professor of climate and space science and engineering at the University of Michigan and deputy principal investigator of the heavy ion sensor.

Electrons in the solar wind are measured by an Electron Analysis System (EAS), developed by UCL’s Mullard Space Science Laboratory, of which Dr Yardley is an honorary member.

Professor Christopher Owen, from UCL, said: “The instrument teams have spent more than a decade designing, building and preparing their sensors for launch, as well as planning how best to operate them in a coordinated manner. So it is very gratifying to now see the data being brought together to reveal which regions of the sun drive the slow solar wind and its variability.

The Proton-Alpha Sensor (PAS), which measures wind speed, has been designed and developed by Paul Sabatier University’s Institut de Recherche en Astrophysique et Planétologie in Toulouse, France.

Together, these instruments make up the Solar Wind Analyzer sensor suite on board Solar Orbiter, for which UCL’s Professor Owen is the principal investigator.

Speaking about future research plans, Dr Yardley said: “So far we have only analyzed Solar Orbiter data in this way for this particular interval. It will be very interesting to look at other cases using Solar Orbiter and also make a comparison with datasets from other close-in missions such as NASA’s Parker Solar Probe.